Rule 1. Use a period at the end of a complete sentence that is a statement.
Example: I know him well.
Rule 2. If the last item in the sentence is an abbreviation that ends in a period, do not follow it with another period.
Incorrect: This is Alice Smith, M.D..
Correct: This is Alice Smith, M.D.
Correct: Please shop, cook, etc. We will do the laundry.
Rule 3. Question marks and exclamation points replace and eliminate periods at the end of a sentence.